Roofing tar is conventional asphalt (or bitumen) and is used as a waterproofing or sealing coating on flat roofs. Most roofing tar is already thinned to a consistency that makes it easy to spread, but if it hasn't been thinned or the can of roofing tar has been left open for too long, you'll need to thin it out before putting it on rollers and running it over the roof.
Warm up the roofing tar to 90 to 100 degrees Fahrenheit; this will reduce its viscosity and make it easier to work with.
Scoop out a cup of roofing tar and put it in a container.
Measure 1/4 cup of water into the container and stir it into the tar.
Insert a tongue depressor or brush into the mix and smear the tar onto a test surface. Roofing tar, for ease of application, should be mildly viscous, somewhere between the viscosity of maple syrup and honey.
Apply more water as needed to the test batch, recording how much water you've added to the tar so you'll have an accurate ratio. Continue to add water, stir, and test the tar until it has the desired consistency.
Write down the ratio, and mix in the appropriate amount of water to the tar bucket. For example, if you determine that you need 1 cup of roofing tar to 3/4 cups of water, your ratio is 1 to 0.75, (or 4 to 3) and you'll put in 3/4 as much water into the mix as you do roofing tar, or 4 parts tar to 3 parts water.